I believe eBay soured for a lot of sellers when they started increasing their fees and changing the fee structure. They always got a cut of course and that's only reasonable, and with tmillions of transactions going on every day they made a tidy profit. Then the fees went up, and a lot of people felt they were being priced out of the market. eBay also own PayPal which is the way most people pay on eBay, so they get an extra cut there.
I'm not an eBay hater btw, just aware that this was a factor. eBay has enabled me to buy things I would never find any other way, and I sold a lot of stuff on eBay just before I moved to the US, and was glad for the site and the big audience I had as a consequence. Sure I could have held a yard sale, garage sale or car boot sale, if I'd had a yard, garage or a car, but I'd never have got the same sale prices.
I think a lot of people run small sales business through eBay, selling collectibles especially which just wouldn't reach a decent market any otherway. It was the first auction site to be useable by the general public (I think), it's the biggest, and it's still a great place to go looking for stuff to buy.